In a non-dispersive infrared (=NDIR) gas analyzer the measurement is based on the absorption of infrared (=IR) radiation in the gas sample. A radiation source directs a beam of infrared radiation through a measuring chamber to a radiation detector whose output signal depends on the strength of the absorption of radiation in the sample gas. The optical wavelength band used for the measurement is selected non-dispersively with an optical bandpass filter. The radiation source typically consists of an electrically heated filament and radiation collecting optics. The gas mixture to be analyzed, i.e. the sample gas, is fed through the measuring chamber, whereupon the gas mixture is contained in the chamber for analysis. The measuring chamber can be a tubular space provided with entrance and exit windows that are transparent at the measurement wavelength and with inlet and outlet for the sample gas. Radiation is absorbed by the gas sample when passing through the measuring chamber.
The radiation detector generates an electric signal that depends on the radiation power falling on its sensitive area. The detector type in a gas analyzer depends on its measurement wavelength, as well as on its construction and operating principle. To make the detector's output signal sensitive to a certain gas component, the wavelength band of radiation coming to the detector is selected so that the gas component absorbs radiation on it. This selection is made using an Optical Bandpass Filter (OBF) whose Bandwidth (BW) is typically 1%–2% of the Center Wavelength (CWL).
In NDIR multigas analyzers, the absorption of the gas sample is measured at several wavelength bands, selected to match the absorption spectra of the gas components of interest. This can be accomplished by using one radiation detector and changing the OBFs on the optical path. It is also possible to use several radiation detectors, combined with their corresponding OBFs. In addition to these measurement detectors, there may be one or more reference detectors. The reference detectors typically receive radiation from the radiation source at wavelength bands where the sample gas is known to have no absorption.
To measure the strength of absorption, it is necessary to know the zero levels of the analyzer at the measurement wavelengths. The zero level is the detector signal obtained at a wavelength when the sample gas does not absorb IR-radiation at that wavelength. The strength of absorption is calculated by forming the ratio between the zero level and the detector signal. Mechanical stresses or shocks, as well as changes in the analyzer's temperature can change the output characteristics of the radiation source and change the zero levels. The sensitivity of IR radiation detectors may depend on their operating temperature, which causes changes to the zero levels. Contamination of the measuring chamber may also change the zero levels. Thus, the zero levels or their estimates must be either continuously measured or updated at regular intervals, typically in the order of some tens of minutes.
It is possible to update the zero levels by zeroing the analyzer. This can be performed by measuring the detector signals when the measuring chamber is filled with a so-called zero gas that is known to not absorb radiation at the measurement wavelengths. The measured zero levels are then used as estimates for the real zero levels until the next zeroing. Zeroings interrupt the analyzer's normal operation typically for several seconds. It is not possible to zero a clinical gas analyzer of the mainstream type because the measuring chamber is all the time located in the patient's breathing circuits, whereupon it is not possible to fill the measuring chamber with zero gas.
It is also possible to obtain estimates for the zero levels without zeroing the analyzer. This can be accomplished by the use of reference filters, whereupon the detector signals are measured at reference wavelengths where the gas sample is known to never absorb IR radiation. It is also possible to use separate reference detectors together with reference filters and use the output signals of the reference detectors as estimates for the zero levels at the measurement wavelengths. These estimates are continuously available, together with the detector signals obtained at the measurement wavelengths. It is often sufficient to use one common reference wavelength for all measurement wavelengths. However, if the measurement wavelengths are widely separated form each other, it may be necessary to use more than one reference wavelength.
A clinically used gas analyzer of the mainstream type is operating on the breathing circuit of the patient, whereupon the whole volume or at least the main portion of the breathing air or gas mixture flows through the analyzer and its measuring chamber. Because the measuring chamber is on the breathing circuit, it is easily contaminated by mucus or condensed water. Thus, it is necessary to use one or more reference wavelengths in a mainstream analyzer in order to have good enough estimate for the zero level continuously available.
A clinical mainstream gas analyzer must be small, light, accurate and reliable. It is not possible to zero it during its normal operation. Yet, the analyzer must maintain its accuracy even if the measuring chamber would be contaminated. Due to these requirements, only single gas analyzers for carbon dioxide CO2 have been available and no multigas analyzers of the mainstream type have been commercially available.
Non-dispersive infrared (NDIR) gas analyzers can be divided to two main types according their optical configuration: dual path analyzers and single path analyzers.
FIG. 8A shows the known principle of the dual path analyzer with one operating wavelength. The analyzer has a radiation source 100, a measuring chamber 101, a reference cell 102, an optical bandpass filter 103, a first radiation detector 105 receiving radiation through the measuring chamber, and a second radiation detector 106 receiving radiation through the reference cell. The reference cell is at all times filled with a gas that has no absorption at the operating wavelength of the analyzer. The essential operating principle of dual path analyzers is that an estimate for the zero level is obtained by blocking the optical path through the measuring chamber and measuring the output signal of the detector when radiation comes to it from the source through the reference cell. In the example of FIG. 8A, this is accomplished by moving the optical bandpass filter 103 to come between the reference cell 102 and the second radiation detector 106. In normal operation, filter 103 is located between the measuring chamber 101 and the first detector 105. The optical and mechanical construction of a dual path analyzer is quite complicated. Problems caused by the contamination of the measuring chamber cannot be solved by the dual path construction. Thus, the dual path construction is generally not used in mainstream gas analyzers.
FIG. 8B shows the known principle of the virtually single path analyzer that can be used as mainstream gas analyzer. The measurement filter 103 has a passband where the gas component of interest absorbs radiation and it is constantly located in front of the measurement detector 105. Filter 103 and detector 105 forms the measurement channel of the analyzer. The reference filter 104 with a passband within which the gas sample does not absorb radiation is located in front of the reference detector 106. Filter 104 and detector 106 forms the reference channel of the analyzer. Radiation form the source 100 passes the measuring chamber 101 and optical filters 103 and 104 and falls on the detectors 105 and 106 respectively. The strength of the absorption can be continuously defined by calculating the ratio between the output signals of the reference and measurement detectors. The benefit of this construction is that it has no moving mechanical parts. Also, signals at both the measurement and reference wavelengths are continuously available. However, the optical paths between the source 100 and the two detectors 105 and 106 are not identical through the measuring chamber. This makes the analyzer sensitive to the contamination of the measuring chamber. The drawback of non-identical optical paths through the measuring chamber can be overcome by using a beam splitter to form a true single beam analyzer, which known alternative is shown in FIG. 8C. Radiation from the source 100 passes the measuring chamber 101 and falls on the beam splitter 107.
Beam splitter 107 transmits part of the radiation to the reference channel formed by second filter 104 and the second detector 106, and reflects part of the incoming radiation to the measurement channel formed by the first filter 103 and the first detector 105. Accordingly, the measurement and reference channels have identical optical paths through the measuring chamber. The drawback of the beam splitter construction is that the beam splitter decreases the radiation input to the detectors and the analyzer's signal to noise ratio gets worse approximately by a factor of two.
U.S. Pat. No. 4,914,719 discloses a single path, multi gas analyzer utilizing a plurality of beam splitters. The analyzer described comprises, for N gases having overlapping absorption spectra where N is an integer greater than 1, a sample cell adapted to contain a gas to be analyzed and a source operative to generate at least one measuring beam which passes through the sample cell, the improvement comprising: means, responsive to the at least one measuring beam, for generating N measuring signals, each indicative of optical energy from the source transmitted through the sample cell in a respective optical region characterized by a respective optical center wavelength λi and a respective bandpass Δλi, where i is an integer greater than 0 and less than or equal to N; means, responsive to the N measuring signals, for combining the N measuring signals to automatically determine which of the N gases is present in the sample cell in the greatest concentration and the concentration thereof. Each of the N measuring signals is indicative of no absorbance in the respective optical region. Said λi and Δλi, are selected such that each of the N measuring signals is a substantially linear function of concentration of each of the N gases in the sample cell. Each of the N gases is characterized by significant absorption in each of the N optical regions. The second means comprises means for algebraically combining the N measuring signals to determine the concentration of each of the N gases in the sample cell. However, this publication does neither discuss at all about the necessity of the reference signal, nor disclose any suggestions for the purpose. The disclosed analyzer uses several beam splitters in series, which causes further decrease of the radiation input to the detectors still worsening the signal to noise ratio of the analyzer.
US patent application 2002/0036266 discloses infrared optical multigas analyzers mainly of the dual path or multi path principle, but also one alternative according to, in a way, the single path principle. The analyzers comprise, in general: an infrared optical radiation source arrangement; a first multispectral detector; a second multispectral detector; a cuvette containing the gas mixture to be measured, said infrared optical radiation source being positioned such that the radiation emitted in a first wavelength range reaches the first multispectral detector through the interior space of the cuvette and radiation emitted in a second wavelength range reaches the second multispectral detector through the interior space of the cuvette, said first wavelength range and said second wavelength range being selected such that they will be different from one another. In the main alternatives of this infrared optical gas analyzer the arrangement comprises a first infrared optical radiation source positioned such that the radiation emitted in the first wavelength range reaches the first multispectral detector through the interior space of the cuvette and second radiation source provided such that the radiation emitted in the second wavelength range reaches the second multispectral detector through the interior space of the cuvette. According to the deviating alternative the infrared radiation emitted by the infrared optical radiation source is passed through an entry window into the cuvette, through the interior space of the cuvette, and then the first part of the radiation goes through the dichroic beam splitter reaching the first multispectral detector from there. The second part of the infrared radiation that is not passed through the dichroic beam splitter is reflected at the dichroic beam splitter and passes from there through the interior space of the cuvette through the exit window, which is transparent to infrared light, to reach the second multispectral detector. This publication, either, does not discuss at all about the necessity of the reference signal, nor disclose any suggestions for the purpose, but is directed to detecting the absorption spectra of the several gas components only. In the disclosed analyzer there is either multiple parallel paths through the cuvette, or multiple serial paths through the cuvette. The latter alternative brings one window surface, which is not common to the path and accordingly a potential source of error caused by contamination, and the surface of dichroic beam splitter towards the interior of the cuvette, which beam splitter surface both reflects and transmits and accordingly is a potential source of error caused by contamination, because contamination of this surface has different effect on the reflected radiation part than on the transmitted radiation part. The path lengths through the cuvette into the first multispectral detector and to the second multispectral detector are different. Accordingly, this deviating alternative is not a genuine single path analyzer.